This invention relates to a light absorbing coating.
It is a thin multi-layer coating, with an absorption function in the visible and/or near infrared spectral range.
The invention is particularly applicable to imagery, to limit parasite reflections and improve the separation between different detection channels, display screens and optical disks in which the function of the coating is to improve the contrast.
The invention is particularly applicable to:
optical systems designed for space applications,
high precision imagery (telemetry),
flat liquid crystal display or microtip display, and
high definition television.
One known method of making an absorbent coating (in the visible-near infrared range) consists of trapping a maximum amount of light that penetrates into the coating, and if possible to limit the amount of reflected light.
In most cases, parasite reflections are restricted by depositing an anti-reflection layer (AR) with a wide spectral range, on an absorbent layer.
The global structure thus defined may be made symmetric if necessary in order to limit light reflections from the substrate side or from the air side.
A distinction is made between three main categories of structures, depending on the required performances:
1) substrate/absorbent layer/air type structures,
2) substrate/absorbent layer/AR layer/air type structures, and
3) substrate/AR layer/absorbent layer/AR layer/air type structures.
A highly reflecting opaque metal layer may be deposited on the substrate beforehand, to limit optical transmission through these structures.
A very wide variety of techniques and materials are used to make absorbent layers, often called xe2x80x9cblack matricesxe2x80x9d.
The most frequent manufacturing method is to use a base layer (of polymer, resin or glass), into which absorbent pigments are added.
These absorbent pigments may for example be compounds based on iron oxide, cobalt aluminate or graphite.
The techniques used to deposit this type of composite absorbent layer very frequently include rolling.
However, there are other methods such as electrodeposition, vacuum deposition, or even anodic oxidation, calcination or laser annealing to improve surface absorption.
One known technique is to use absorbent layers made of black chromium obtained by electrodeposition making use of an electrolytic bath with a composition that is adjusted so that the color obtained is black.
This subject is described in document 1, which like other documents referenced later, is mentioned in the references at the end of this description.
We will consider vacuum deposition techniques in particular.
In this case, absorbent layers are generally inserted in an interference structure with thin stacked layers based on dielectric materials, particularly oxides.
There are then structures type 2) and type 3).
The antireflection function may be provided conventionally by multiple layers, with alternating high and low optical indexes n.
This is done for example using TiO2 layers (n is approximately equal to 2.4) and SiO2 layers (n is approximately equal to 1.5), or MgF2 layers (n is approximately equal to 1.39).
The thickness of these layers is of the order of a few tenths of a micrometer, and the way in which they are stacked controls the spectral width and the central wave length of the band or antireflection function.
Several solutions have been proposed for the manufacture of the actual absorbent layers.
Absorbent layers frequently consist of oxides which are naturally absorbent or which are made absorbent by creating an oxygen deficiency at the time of deposition (sub-stoechiometric oxides).
For example, inherently absorbent compounds may include indium oxide, tungsten oxide, chromium oxide, tin oxide (see document 2), or even vanadium oxide.
Among these various compounds, note in particular that CrO2, In2O and SnO oxides are black.
Concerning sub-stoechiometric oxides, most compounds that are transparent in their stable form, may become absorbent if an oxygen depletion is created in them.
For example, this is the case for NiOx nickel oxides recommended in document 3, which are inserted in a double TiO2/SiO2/NiOx/TiO2/SiO2 type of anti-reflection structure.
Note also the structure proposed in document 4, which is composed of a multiple layer stack consisting of Cr/absorbent dielectric/AR layer.
The absorbent layer is then preferably composed of manganese oxide, or chromium or iron oxide, or possibly silica containing dispersed chromium.
Other materials such as nitrides (TiN and ZrN mentioned in document 3) may also form useful absorbent layers.
In particular, it is well known that titanium nitride layers TiNx offer a wide range of green-bronze to golden yellow colors, depending on the value of x.
Note also the use of more exotic absorbent materials such as the SiGe compound (see document 5).
Note also the absorbent stack Cr/Cr2O3/Cr/Cr2O3 that is used to improve the contrast of liquid crystal screens (see document 6), and which includes layers with a thickness varying from 10 nm to a few tens of nanometers.
Light is generally absorbed using more or less absorbent materials, in which the extinguishing coefficient k normally remains less than or equal to about 10xe2x88x921 in the visible-near infrared range (k being of the order of 10xe2x88x924 to 10xe2x88x925 for a transparent dielectric).
In this case, if it is required to make a layer with quasi-total absorption, a layer must be used with a thickness equal to at least 1 xcexcm, and it must be provided with anti-reflection layers to limit reflections at the air/layer interface.
A better way of maximizing absorption is to use an absorbent structure formed of N layers of Cr alternating with N layers of Cr2O3, given the high extinguishing coefficient k of chromium (k is equal to 4 in the visible range).
Absorption of at least 95% of the incident light in the visible range then requires that several Cr/Cr2O3 pairs are stacked (N greater than 2), hence the total thickness of chromium and chromium oxide is at least 100 nm to 200 nm.
The thickness of this structure must increase as the required absorption gets closer to 100%.
The performance is then limited due to mechanical strength problems.
It is well known that thin layers of chromium are some of the most highly stressed metals, which typically limits its thickness to 200 nm (see document 7).
Above this thickness the layer crazes and separates.
Therefore, guaranteeing mechanical stability of the absorber requires that the thinnest possible multilayer structure should be used, and using materials with low stresses.
The purpose of this invention is to design a multilayer coating with a high absorption capacity (absorption of more than 95% of light) in the visible near infrared range, while minimizing risks of mechanical instability related to stresses in the layers in this absorbent coating.
The invention achieves this using an absorbent multi-layer structure that includes at least one thin metal layer as an absorption element, with the specific feature that it is optically discontinuous.
In particular, as we will see in the examples given later, this restricts the total thickness of the absorber to less than 200 nm, while providing absorption of between 97% and 99% within the visible-near infrared range.
Specifically, the purpose of the invention is a light absorbing coating in a given spectral range within the visible-near infrared range, this coating being formed on a substrate and characterized in that it comprises:
at least one layer of thin metal which is absorbent in this determined spectral range, and
at least one dielectric layer which is transparent in this determined spectral range, this dielectric layer being formed on this thin metal layer,
and in that this thin metal layer is optically discontinuous, its refraction index being greater than the refraction index of the metal in the bulk state, and its extinguishing coefficient being less than the extinguishing coefficient of the metal in the bulk state, within the determined spectral range.
xe2x80x9cMetal in the bulk statexe2x80x9d means a very thick layer of this metal with a thickness of at least 1 xcexcm.
According to one specific embodiment of the coating according to the invention, in order to increase the absorption of this coating, this coating comprises at least two thin, optically discontinuous metal layers, the dielectric layer being formed on the thin metal layer furthest from the substrate.
The coating according to the invention may also comprise a thick, optically continuous metal layer between the substrate and the thin metal layer closest to this substrate.
Within this spectral range concerned, the refraction index of this thick layer is then approximately equal to the refraction index of the corresponding metal in the bulk state, and the extinguishing coefficient of this thick layer is approximately equal to the extinguishing coefficient of the corresponding metal in the bulk state.
The function of this optically continuous metal layer formed on the substrate is to prevent light transmission and to reflect light towards the absorbent layer(s).
When this optically continuous metal layer is made of aluminum, a transmission coefficient of less than 10xe2x88x923 in the visible range is obtained when the aluminum thickness is not less than 50 nm.
A coating according to the invention can be made which is xe2x80x9csymmetricxe2x80x9d, so that it performs the same optical function on the substrate side and on the air side; a coating according to the invention may comprise, in sequence starting from the substrate, a first dielectric layer, a first optically discontinuous thin metal layer, a second dielectric layer, an optically continuous thick metal layer, a second optically discontinuous thin metal layer, and a third dielectric layer.
The optically continuous thick metal layer is preferably a metal M having a property that its oxide MxOy which is transparent within the determined spectral range.
This metal in the optically continuous thick metal layer may be chosen from the group including titanium, hafnium, chromium and niobium but aluminum will be used in preference, since it has a high extinguishing coefficient k (of the order of 5 to 8 in the visible range).
In a coating according to the invention, each dielectric layer may be chosen from oxide layers that are dielectric and transparent in the determined spectral range.
However, compounds other than these oxides may be used, if necessary for the application.
For example, it would be possible to use dielectric fluorides that are transparent within the determined spectral range.
According to one embodiment of the invention, preferred for its performances, each optically discontinuous thin layer is an aluminum layer with a thickness less than or approximately equal to 15 nm, the coating also comprising an alumina layer on which this thin, optically discontinuous layer is formed.
Note that the choice of the stack to be made to obtain a coating according to the invention depends on the spectral band width at which maximum absorption is required.
This choice also depends on the residual reflection level tolerated in the application considered.
Therefore one of the features of this invention is that the optical performances of the coating are adjustable depending on the specific requirements.
In the case of the preferred embodiment mentioned above, a coating according to the invention of the following type can be made:
substrate/optically continuous metal layer/alumina layer/optically discontinuous aluminum layer/dielectric layer.
A coating of the following type can also be made according to the invention to increase optical absorption:
substrate/optically continuous metal layer/alumina layer/optically discontinuous aluminum layer/ . . . /alumina layer/optically discontinuous aluminum layer/dielectric layer
in which the xe2x80x9calumina layer/optically discontinuous aluminum layerxe2x80x9d pattern is repeated at least twice.
A coating according to the invention with a xe2x80x9csymmetricxe2x80x9d type structure of the following type can also be made:
substrate/dielectric layer/alumina layer/optically discontinuous aluminum layer/dielectric layer/optically continuous metal layer/alumina layer/optically discontinuous aluminum layer/dielectric layer.
Coatings conform with the invention will preferably be made by vacuum deposition of the layers.
Typical thicknesses of these layers are a few nanometers for an optically discontinuous aluminum layer and a few tens or a few hundreds of nanometers for each dielectric layer such as Al2O3 and for each optically continuous metal layer.
A slow and stable deposition technique must be used to control the small thicknesses of metal.
The ionic atomization (cathodic or by ion beam) method is particularly well adapted to the invention.
The advantage of using a homogeneous xe2x80x9calumina layer/optically discontinuous aluminum layerxe2x80x9d type of absorbent structure is that the changeover from the metal layer to the oxide layer can be made simply be adding oxygen into the containment in which the coating is being formed.
In this case, the layer deposition device is particularly simple, since all it comprises is one metallic atomization target placed in a vacuum containment in which atomization is done successively in a rare gas atmosphere and in a mix of rare gases and oxygen.
One of the innovative features of this invention lies in the use of one or several thin, optically discontinuous metal layers, in which the optical constants n and k are very different from those of bulk metal, or from those of a thick layer of this metal (which are not very different from the values for the bulk metal).
Document 8 contains information about the optical constants for a thick layer of aluminum.
The use of at least one thin optically discontinuous metal layer induces high absorption which is significantly higher than if an optically continuous metal layer is used.
Another advantage of a coating according to the invention is that it is thin.
The preferred use of aluminum and alumina also restricts mechanical stresses in the layers of the coating.
Consequently, a perfectly bonding structure is obtained, to which subsequent treatment can be applied without damage.
For example, it is possible to perform ionic etching or xe2x80x9clift-offxe2x80x9d type etching of openings in the coating and thus identify the boundaries of local diaphragms.
Heat treatments can also be applied at temperatures up to the order of 300xc2x0 C. to 400xc2x0 C. without significantly modifying the absorption function, the alumina acting as a diffusion barrier.
Another advantage resulting from the low thickness of a coating according to the invention is the limitation of surface diffusion that is typically less than 1% in the visible range.
This is important when the objective is to limit parasite light in optical systems.